Melt butter in a small saucepan on medium heat. Add in the flour. Stir or whisk constantly. Cook for about 30 seconds. Remove from the heat. . Pour in the chicken broth and whisk until there are no lumps. Return the saucepan to the stove and lower the heat. Allow to simmer for 2-3 minutes, or until mixture begins to thicken. Add in half and half, milk, thyme, garlic powder, and pepper. Allow the sauce to simmer for 3 minutes more. Remove from the heat and allow the sauce to cool slightly. **Sauce will thicken more as it begins to cool.

Cook the elbow macaroni according to package instructions. Drain. Preheat the oven to 350. Grease a 9x13 baking dish with butter or cooking spray. Add the shredded chicken to the bottom of the baking dish. Then top with pasta, mixed vegetables, water chestnuts, and seasonings. Place spoonfuls of the sour cream over the top. Pour the creamy sauce over the top. Stir until everything is combined. Sprinkle with cheddar cheese. Combine Cornflakes with butter in a small bowl. Sprinkle the cornflake mixture over the top of the casserole. Bake in the oven for 1 hour or until casserole is heated through and golden brown.